Conventionally, waveform equalization circuits, error correction circuits, and/or the like have been used to process small amplitude signals with high accuracy. In the field of optical disks, Partial Response Maximum Likelihood (PRML) read channel technology is generally used to improve the accuracy of data reading. In this technology, a waveform equalization process, which allows the waveform of a digital signal to become a predetermined waveform using a digital waveform equalization circuit such as a transversal filter, and a maximum-likelihood decoding process, which performs error correction by using a maximum-likelihood decoding method for the digital signal after the waveform equalization process, are performed. For example, in Patent Document 1, an analog filter capable of amplifying an amplitude in a particular frequency band corrects the waveform of an reproduced signal (an analog signal) reproduced from an optical disk, an analog-to-digital (A/D) converter converts the corrected reproduced signal into a digital signal, and a transversal filter and a maximum-likelihood decoder respectively perform the waveform equalization process and the maximum-likelihood decoding process. Thus, in Patent Document 1, an analog waveform equalization circuit is used to correct amplitudes of analog signals supplied to an A/D converter, and the transversal filter (an digital waveform equalization circuit) is used for waveform equalization process of digital signals. Additionally, Japanese Unexamined Patent Application Publication No. 563-234730 discloses technologies for error detection and error correction within an A/D converter.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-8315    Patent Document 2: Japanese Unexamined Patent Application Publication No. S63-234730